Transmission between satellites and earth stations is established by means of antennas which are either linearly or circularly polarized. In order to optimize satellite communication links, it is essential that the polarization of the earth station antenna be matched to the polarization of the satellite antenna. Thus, if circular polarization is employed on the satellite this is also the optimum for the earth station, and similarly for linear polarization. In many modern satellite communications systems the limited frequency resource is most efficiently used by employing dual, orthogonal polarization, and thus antenna polarization characteristics are of utmost importance in such systems.
If, e.g., a circularly polarized earth station antenna is being employed to receive a linearly polarized satellite signal, there will be a 3-dB power loss associated with the link due to the polarization mismatch. Moreover, if the satellite operates in dual-linear polarization, the interference between the two corresponding signals will be such as to prevent useful satellite communication.
If the above problems could be alleviated, earth stations, which often represent a significant investment, could be used for satellites other than those they were originally intended for, regardless of the polarization scheme. Additionally, if these problems could be solved in such a manner that alternative operation of an earth station in either circular or linear polarization is possible without significant additional cost, earth stations could be more readily designed to accommodate operations with both circularly and linearly polarized satellites. It would thus give the satellite operator greater flexibility, since satellites of differing polarization schemes could be substituted for each other during the life of the earth station without degradation of communications.
It is therefore an object of the present invention to provide a technique by which single- or dual-circularly polarized earth stations can be retro-fitted to access linearly polarized satellites with inexpensive and easily installed modifications to the existing earth station hardware, which are easily removed to recover the original configuration.
It is a further object of this invention to provide a means which allow, without significant additional expense or operational difficulty, earth stations to be designed and manufactured to accommodate either circularly or linearly polarized communications at any given time.